RS Address Format

As of version 1.0.1 of the Nxt software, the use of Reed-Solomon addresses for Nxt account is supported. It's the default format in the official client.

This form of address improves reliability by introducing redundancy that can detect and correct errors when entering and using Nxt account numbers.

These addresses are always in the form of:

NXT-XXXX-XXXX-XXXX-XXXXX

... where X is a non-ambiguous number or alphabetic character (the letters O and I are not used; nor are the numbers 1 and 0). Addresses are always prefixed with "NXT-", and hyphens are used to separate the address into groups of 4, 4, 4, and then 5 characters. The addresses are NOT case-sensitive.

Contents

1 Background

The default format for Nxt account numbers is a completely numeric 64-bit identifier that is derived from the account's private key. Early adopters of Nxt complained that this format was error-prone, since there were numerous instances where a single mistyped character resulted in transactions being unintentionally sent to the wrong account.

2 Encoding of Nxt Reed-Solomon addresses

Case is not enforced in this format, but for unification all addresses are displayed using upper case.

Addresses are split by dashes into groups of 4 characters and a final group of 5 characters, but this is not enforced during address input.

The old numeric addresses are also recognized and supported for backwards compatibility.

Example RS Addresses:

NXT-3DH5-DSAE-4WQ7-3LPSE

NXT-K4G2-FF32-WLL3-QBGEL

3 Technical Details

The first and most important rule is that no error-correction scheme is infallible: You cannot rely on error correction, period.

The problem is somewhat counter-intuitive: either you can do a simple yes/no check of address validity, which will give you one in a million collision, or you can try and correct errors. You cannot do both.

The problem here is that the Reed-Solomon algorithm is only guaranteed to correct up to 2 errors. If there are more than 2 errors present in an address entry, it will produce false positives with a probability of around 10% and transactions will still be sent to incorrect addresses.

Think of the algorithm as error-guessing, instead, to assist users with spotting errors.

Reed-Solomon (RS) addresses for Nxt are encoded as follows:

Take the original 64-bit account ID, add 1 zero bit to get 65 and then split it into thirteen 5-bit "symbols" (65 / 5 = 13).

Order the symbols from lowest bit to highest bits, in little-endian order, i.e. bits 0-4, 5-9, 10-14, etc. up to 60-64.

Append 4 symbols of parity (20 bits), produced by the Reed-Solomon encoding of our 13 symbols from step one (which are left untouched). This produces a 13 + 4 = 17 symbol codeword.

Scramble the codeword symbols in a predefined order and encode them 1-to-1 with an alphabet of 32 characters, splitting them into groups by dashes.